1. Field of the Invention:
This invention relates to photopolymerizable compositions. More particularly, this invention relates to compositions which comprise an organic, cationically polymerizable material and an aromatic iodonium complex salt as photoinitiator and which can be cured by exposure to actinic radiation or electron beam irradiation.
2. Description of the Prior Art:
Although photopolymerizable epoxy compositions comprising epoxy resin materials and photosensitizers have been desired for some time, those which have previously been proposed suffer from one or more drawbacks. Thus, in U.S. Pat. No. 3,074,869 there are disclosed photosensitive epoxy compositions containing a nitrosoamine as photosensitizer. Compositions of this type require relatively long exposure to a high intensity light source to produce complete polymerization.
In U.S. Pat. Nos. 3,205,157 and 3,708,296, there are disclosed photosensitive epoxy compositions containing respectively aryldiazonium salts and aryldiazonium salts of halogen-containing complex anions. Such compositions have limited usefulness because they have poor thermal stability, because their spectral response is limited to the ultraviolet region of the spectrum, and because nitrogen is evolved during photopolymerization causing pinholes and bubbles in heavy coatings of the composition.
When these known aryldiazonium salts are used to induce polymerization of oxetanes, or mixtures of oxetanes with epoxy resins, e.g., as described in U.S. Pat. No. 3,835,003, the same types of problems are encountered. Although several patents describe various techniques for stabilizing mixtures of diazonium salts and epoxides, such techniques are not satisfactory for several reasons. For example, the increase in stability which is obtained is measured only in days. Also, the addition of stabilizers contaminates the compositions with nonreactive material which softens the resulting product and also reduces the rate of photocure. See, e.g., U.S. Pat. Nos. 3,711,390; 3,711,931; 3,816,278; 3,816,280; 3,816,281; 3,817,850; and 3,817,845.
In U.S. Pat. No. 3,450,613 there is described another photopolymerizable epoxy composition comprising the reaction product of an epoxy resin prepolymer and an ethylenically unsaturated organic acid, a photosensitizer, and optionally polyfunctional acids or bases. This composition on exposure to ultraviolet light gels by reason of the photoinduced polymerization of the ethylenically unsaturated portion of the reaction product. Completion of the cure of the composition is effected by heating to bring about reaction of the epoxy resin portion of the composition. Such compositions as this have limited usefulness because of the requirement of both light and heat to effect complete polymerization of the composition. The composition furthermore is oxygen sensitive and has poor thermal stability.
The present invention provides novel photopolymerizable compositions, comprising cationically polymerizable materials and a photoinitiator, which are sensitive throughout the ultraviolet and visible spectral regions, i.e., wavelengths from about 300 to 700 millimicrons, and which may be photocured by exposure for relatively short periods of time to radiation within this range of wavelengths or by exposure to electron beam irradiation. The invention also provides novel complex salt photoinitiators.